Regulation of Respiration In Nervous Physiology Notes

Regulation of Respiration Introduction

• Respiration is a reflex process. But it can be controlled voluntarily.
• Voluntary arrest of respiration (voluntary apnea) is possible but only for a short period of about 40 seconds.
• However, by practice, breathing can be withheld for a long period.
• At the end of that period, the person is forced to breathe. Respiration is subjected to variation even under normal physiological conditions.
• Emotion and exercise increase the rate and force of respiration. Rest and sleep decrease the rate and force of respiration.
• However the altered pattern of respiration is brought back to normal within a short time by some regulatory mechanisms in the body.

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The pattern of respiration is regulated by two mechanisms:

1. Nervous or neural mechanism
2. Chemical mechanism.

Nervous Mechanism

The nervous mechanism regulates respiration by the reflex process. This mechanism includes respiratory centers, afferent nerves, and efferent nerves.

Respiratory Centers: Respiratory centers are groups of neurons, which control the rate, rhythm, and force of respiration.

These centers are bilaterally situated in the reticular formation of the brainstem. Depending upon the situation in the brainstem, the respiratory centers are classified into two groups:

1. Medullary centers which are made up of
2. The dorsal respiratory group of neurons
3. Ventral respiratory group of neurons T Pontine centers which are
4. Pneumotaxic center
5. Apneustic center.

Medullary Centers:

Dorsal Respiratory Group of Neurons Situation:

• The dorsal respiratory group of neurons is diffusely situated in the nucleus of tractus solitarius which is present in the upper part of the medulla oblongata.
• Formerly these neurons were collectively called inspiratory centers.
• All the neurons of the dorsal respiratory group are inspiratory neurons that generate inspiratory ramp by virtue of their autorhythmic property.

Function: The dorsal group of neurons is responsible for the basic rhythm of respiration (see below for details).

Experimental evidence:

• Electrical stimulation of these neurons in animals by using needle electrodes causes contraction of inspiratory muscles and prolonged inspiration.

Ventral Respiratory Group of Neurons:

Situation :

• The ventral respiratory group of neurons is present in the nucleus ambiguous and the nucleus retro ambiguous.
• These two nuclei are situated in the medulla oblongata anterior and lateral to the nucleus of tractus solitarius.
• Earlier the ventral group neurons were collectively called expiratory centers.
• The ventral group has both inspiratory and expiratory neurons. The inspiratory neurons are found in the central area of the group.
• The expiratory neurons are in the caudal and rostral areas of the group.

Function:

• Normally, the ventral group neurons are inactive during quiet breathing and become active during forced breathing.
• During forced breathing, these neurons stimulate both inspiratory muscles and expiratory muscles.

Experimental evidence:

• Electrical stimulation of the inspiratory neurons in the ventral group causes contraction of inspiratory muscles and prolonged inspiration.
• Stimulation of expiratory neurons causes contraction of expiratory muscles and prolonged expiration.

Pontine Centers:

Pneumotaxic Center:

Situation:

• The pneumatic center is situated in the dorsolateral part of the reticular formation in the upper pons.
  It is formed by the neurons of medial parabrachial and sub-parabrachial nuclei.
• The sub-parabrachial nucleus is also called ventral parabrachial or Kolliker-Fuse nucleus

Function:

• The primary function of the pneumatic center is to control the medullary respiratory centers, particularly the dorsal group neurons.
• It acts through an apneustic center. The pneumatic center inhibits the apneustic center so that the dorsal group neurons are inhibited.
• Because of this inspiration stops and expiration starts.
• Thus, the pneumatic center influences the switching between inspiration and expiration.
• The pneumatic center increases the respiratory rate by reducing the duration of inspiration.

Experimental evidence:

• Stimulation of the pneumatic center does not produce any typical effect, except slight prolongation of expiration by inhibiting the dorsal respiratory group of neurons through the apneustic center.
• Destruction or inactivation of pneu-motaxic center results in apneusis.
• Apneusis is an abnormal pattern of respiration or breathing irregularity characterized by prolonged inspiration followed by short, inefficient expiration.

Apneustic Center:

Situation:

1. The apneustic center is situated in the reticular formation of lower pons.
2. Respiratory System and Environmental Physiology

Function: This center increases the depth of inspiration by acting directly on the dorsal group neurons.

Experimental evidence: The stimulation of the apneustic center causes apneusis.

Connections Of Respiratory Centers:

Efferent Pathway:

• The nerve fibers from the respiratory centers leave the brainstem and descend in the anterior part of the lateral columns of the spinal cord.
• These nerve fibers terminate on the motor neurons in the anterior horn cells of the cervical and thoracic segments of the spinal cord. From the motor neurons of the spinal cord, two sets of nerve fibers arise: Phrenic nerve fibers (C₃ – C₅) which supply the diaphragm
  The intercostal nerve fibers (T₁ – T₁₁) supply the external intercostal muscles.
• The Vagus nerve also contains some efferent fibers from the respiratory centers.

Afferent Pathway: Impulses from peripheral chemoreceptors and baroreceptors are carried to the respiratory centers by the branches of glossopharyngeal and vagus nerves.

• Vagal nerve fibers also carry impulses from the stretch receptors of the lungs to the respiratory centers.
• Thus, the respiratory centers receive afferent impulses from different parts of the body and, modulate the movements of the thoracic cage and lungs accordingly through efferent nerve fibers.

Integration Of Respiratory Centers:

Role of Medullary Centers:

• The rhythmic discharge of inspiratory impulses Dorsal respiratory group neurons are responsible for the normal rhythm of respiration.
• These neurons maintain the normal rhythm of respiration by rhythmic discharge of impulses (action potentials). These impulses are transmitted to the respiratory muscles by the fibers of phrenic and intercostal nerves.

Inspiratory ramp:

• The inspiratory ramp is the pattern of discharge from dorsal respiratory group neurons characterized by a steady increase in the amplitude of the action potential.
• The firing of these neurons is not like a sudden outburst and the discharge is also not uniform. To start with, the amplitude of the action potential is low.
• It is due to the activation of only a few neurons.
• Later, more and more neurons are activated leading to a gradual increase in the amplitude of the action potential in a ramp fashion. The impulses of this type of firing from dorsal group neurons are called inspiratory ramp signals.
• The impulses from dorsal group neurons are not produced continuously but only for a period of 2 seconds during which inspiration occurs.
• After 2 seconds, the ramp signals stop abruptly and do not appear for another 3 seconds. The switching off-ramp signals cause expiration.
• At the end of 3 seconds, the inspiratory ramp signals reappear in the same pattern, and the cycle is repeated.
• Normally, during inspiration, the dorsal respiratory group neurons inhibit the expiratory neurons of the ventral group.
• During expiration, the expiratory neurons inhibit the dorsal group neurons.
  Thus, the medullary respiratory centers control each other.

Significance of inspiratory ramp signals:

• The significance of inspiratory ramp signals is that there is a slow and steady inspiration so that, the filling of lungs with air is also steady.

Role of Pontine Centers:

• Pontine respiratory centers regulate the medullary centers. The apneustic center accelerates the activity of dorsal group neurons and the stimulation of this center causes prolonged inspiration.
• The pneumatic center inhibits the apneustic center and restricts the duration of inspiration.

Pre-Botzinger Complex:

• Recently scientists have discovered another center for respiration in animals. It is called preBotzinger complex (preBotC).
• This center is formed by a group of neurons located in the ventrolateral part of the medulla.
• It is suggested that these neurons are the pacemaker neurons and generate rhythmic respiratory impulses.
• It is also suggested that the neurons of medullary centers are projected into this complex.
• The exact functioning mechanism of this center is not known.

Factors Affecting Respiratory Centers:

The respiratory centers regulate the respiratory movements, by receiving impulses from various sources in the body.

Impulses from Higher Centers:

• Higher centers alter respiration by sending impulses directly to the dorsal group neurons.
• The impulses from the anterior cingulate gyrus, genu of corpus callosum, olfactory tubercle, and posterior orbital gyrus of cerebral cortex inhibit respiration.
• The impulses from the motor area and Sylvian area of the cerebral cortex cause forced breathing.

Impulses from Stretch Receptors of Lungs:

Hering-Breuer Reflex:

• The hearing-Breuer reflex is a protective reflex that restricts inspiration and prevents the over-stretching of lung tissues. It is initiated by the stimulation of stretch receptors of air passage.
• Stretch receptors are the receptors that give a response to the stretch of the tissues. These receptors are situated on the wall of the bronchi and bronchioles.
• During inspiration, the lungs expand. This causes stretching of the lungs and the air passage. So the stretch receptors are stimulated.
  The impulses from stretch receptors are transmitted by vagal afferent fibers to the respiratory centers.
• The impulses actually inhibit the dorsal group neurons and so inspiration stops and expiration starts.
• Thus, the overstretching of lung tissues is prevented. However, the Hering-Breuer reflex does not operate during quiet breathing. It operates, only when the tidal volume increases beyond 1000 mL.
• This reflex is also called the Hering-Breuer inflation reflex since it restricts the inspiration and limits the over-stretching of lung tissues.
• The reverse of this reflex is called the Hering-Breuer deflation reflex and it takes place during expiration. During expiration, as the stretching of the lungs is abolished, the deflation of the lungs occurs.

Impulses from ‘J’ Receptors of Lungs:

• ‘J’ receptors are juxtacapillary receptors that are present on the wall of the alveoli and have close contact with the pulmonary capillaries.
• AS Paintal found that these receptors are the sensory nerve endings of the vagus.
  The fibers from these receptors are nonmyelinated and belong to the C type. Few receptors are found on the wall of the bronchi.

The ‘J’ receptors are stimulated during the following conditions:

1. Pulmonary congestion
2. Pulmonary edema
3. Pneumonia
4. Overinflation of lungs
5. Microembolism in pulmonary capillaries.

Some exogenous and endogenous chemical substances like histamine, halothane, bradykinin, serotonin, and phenyldiguanide also stimulate the ‘J’ receptors.

• The stimulation of the ‘J’ receptors produces a reflex response, which is characterized by apnea.
• Apnea is foHsv/ed by hyperventilation, bradycardia, hypotension, and weakness of skeletal muscles.
• The role of ‘J’ receptors in physiological conditions is not clear.
• However, these receptors are responsible for hyper-ventilation in patients affected by pulmonary congestion and left heart failure.

Impulses from Irritant Receptors of Lungs:

• Besides stretch receptors, there is another type of receptor in the bronchi and bronchioles lungs, called irritant receptors.
• The irritant receptors are stimulated by irritant chemical agents such as ammonia and sulfur dioxide.
• These receptors send afferent impulses to respiratory centers via vagal nerve fibers.
• Stimulation of irritant receptors produces reflex hyper-ventilation along with bronchospasm.
  Hyperventilation along with bronchospasm prevents further entry of harmful agents into the alveoli.

Impulses from Baroreceptors:

• The baroreceptors are the receptors which give respond to a change in blood pressure. These receptors are also called pressoreceptors.

Function:

• The baroreceptors in the carotid sinus and arch of the aorta respond to an increase in blood pressure.
• Whenever arterial blood pressure increases, baroreceptors are activated and send inhibitory impulses to the medulla oblongata. This causes a decrease in blood pressure and inhibition of respiration.
• However, in physiological conditions, the role of baroreceptors in the regulation of respiration is insignificant.

Impulses from Chemoreceptors:

• Chemoreceptors play an important role in the chemical regulation of respiration.
• The details of the chemo-receptors and chemical regulation of respiration are explained later in this chapter.

Impulses from Proprioceptors:

• Proprioceptors are the receptors, that give response to the change in the position of the body. These receptors are situated in joints, tendons, and muscles.
• The proprioceptors are stimulated during muscular exercise and, send impulses to the brain particularly, the cerebral cortex through somatic afferent nerves.
• The cerebral cortex in turn causes hyperventilation by sending impulses to the medullary respiratory centers.

Impulses from Thermoreceptors:

• Thermoreceptors are the cutaneous receptors, which; give respond to changes in the environment temperature.
• There are two types of temperature receptors, namely, the receptors for cold and the receptors for warmth.
• When the body is exposed to cold or when cold water is applied over the body, the cold receptors are stimulated and, send impulses to the cerebral cortex via somatic afferent nerves.
• The cerebral cortex in turn stimulates the respiratory centers and causes hyperventilation.

Impulses from Pain Receptors:

• The pain receptors are those which give a response to pain stimulus.
• Whenever pain receptors are stimulated, the impulses are sent to the cerebral cortex via somatic afferent nerves.
• The cerebral cortex in turn stimulates the respiratory centers and causes hyperventilation.

Chemical Mechanism

• The chemical mechanism of regulation of respiration is operated through the chemoreceptors.
• The chemoreceptors are the sensory nerve endings, which respond to chemical changes in the blood.

The chemoreceptors are stimulated by the changes in the chemical constituents of the blood such as:

1. Hypoxia (decreased P02)
2. Hypercapnia (increased PC02)
3. Increased hydrogen ion concentration.

Types of Chemoreceptors:

Chemoreceptors are classified into two groups:

1. Central chemoreceptors
2. Peripheral chemoreceptors.

Central Chemoreceptors:

The chemoreceptors present in the brain are called the central chemoreceptors.

Situation:

• Central chemoreceptors are situated in the deeper part of the medulla oblongata, close to the dorsal respiratory group of neurons.
• This area is known as the chemosensitive area and the neurons are called chemoreceptors. The chemoreceptors are in close contact with blood and cerebrospinal fluid.

Mechanism of Action:

• The central chemoreceptors are connected with respiratory centers particularly the dorsal respiratory group of neurons through synapses.
• These chemo-receptors act slowly but effectively. The central chemo-receptors are responsible for 70-80% of increased vernation through the chemical regulatory mechanism.
• The main stimulant for the central chemoreceptors is the increased hydrogen ion concentration.
• However, if hydrogen ion concentration increases in the blood, it cannot stimulate the central chemoreceptors because the hydrogen ions from the blood cannot cross the blood-brain barrier and blood-cerebrospinal fluid barrier.
• On the other hand, if carbon dioxide increases in the blood, it can easily cross the blood-brain barrier and blood-cerebrospinal fluid barrier and enter the interstitial fluid of the brain or the cerebrospinal fluid.
• There, the carbon dioxide combines with water to form carbonic acid. Since carbonic acid is unstable, it immediately dissociates into hydrogen ions and bicarbonate ions.

CO₂ + H₂O → H₂CO₃ —> H⁺ + HCO^–₃

• The hydrogen ions stimulate the central chemo-receptors. From chemoreceptors, the stimulatory impulses are sent to the dorsal respiratory group of neurons causing increased ventilation (increased rate and force of breathing).
• Because of this, the excess carbon dioxide is washed out and the respiration is brought back to normal.
• Lack of oxygen does not have a significant effect on the central chemoreceptors except that it generally depresses the overall function of the brain.

Peripheral Chemoreceptors: Chemoreceptors present in the carotid and aortic region are called peripheral chemoreceptors.

Mechanism of Action:

• Reduction in partial pressure of oxygen is the most potent stimulant for the peripheral chemoreceptors.
• Whenever the partial pressure of oxygen decreases, the chemoreceptors are stimulated and send impulses through the aortic and Hering’s nerves.
• These impulses reach the respiratory centers, particularly the dorsal group of neurons, and stimulate them.
• The dorsal group of neurons sends stimulatory impulses to respiratory muscles resulting in increased ventilation.
• This provides enough oxygen and rectifies the lack of oxygen. The peripheral chemoreceptors are mildly sensitive to the increased partial pressure of carbon dioxide and increased hydrogen ion concentration.